Earth-boring bits may have fixed or rotatable cutting elements. Earth-boring bits with fixed cutting elements typically include a bit body machined from steel or fabricated by infiltrating a bed of hard particles, such as cast carbide (WC+W2C), tungsten carbide (WC), and/or sintered cemented carbide with a binder such as, for example, a copper-based alloy. Several cutting inserts are fixed to the bit body in predetermined positions to optimize cutting. The bit body may be secured to a steel shank that typically includes a threaded pin connection by which the bit is secured to a drive shaft of a downhole motor or a drill collar at the distal end of a drill string.
Steel bodied bits are typically machined from round stock to a desired shape, with topographical and internal features. Hard-facing techniques may be used to apply wear-resistant materials to the face of the bit body and other critical areas of the surface of the bit body.
In the conventional method for manufacturing a bit body from hard particles and a binder, a mold is milled or machined to define the exterior surface features of the bit body. Additional hand milling or clay work may also be required to create or refine topographical features of the bit body.
Once the mold is complete, a preformed bit blank of steel may be disposed within the mold cavity to internally reinforce the bit body and provide a pin attachment matrix upon fabrication. Other sand, graphite, transition or refractory metal-based inserts, such as those defining internal fluid courses, pockets for cutting elements, ridges, lands, nozzle displacements, junk slots, or other internal or topographical features of the bit body, may also be inserted into the cavity of the mold. Any inserts used must be placed at precise locations to ensure proper positioning of cutting elements, nozzles, junk slots, etc., in the final bit.
The desired hard particles may then be placed within the mold and packed to the desired density. The hard particles are then infiltrated with a molten binder, which freezes to form a solid bit body including a discontinuous phase of hard particles within a continuous phase of binder.
The bit body may then be assembled with other earth-boring bit components. For example, a threaded shank may be welded or otherwise secured to the bit body, and cutting elements or inserts (typically cemented tungsten carbide, or diamond or a synthetic polycrystalline diamond compact (“PDC”)) are secured within the cutting insert pockets, such as by brazing, adhesive bonding, or mechanical affixation. Alternatively, the cutting inserts may be bonded to the face of the bit body during furnacing and infiltration if thermally stable PDCs (“TSP”) are employed.
Rotatable earth-boring bits for oil and gas exploration conventionally comprise cemented carbide cutting inserts attached to cones that form part of a roller-cone assembled bit or comprise milled teeth formed in the cutter by machining. The milled teeth are typically hardfaced with tungsten carbide in an alloy steel matrix. The bit body of the roller cone bit is usually made of alloy steel.
Earth-boring bits typically are secured to the terminal end of a drill string, which is rotated from the surface or by mud motors located just above the bit on the drill string. Drilling fluid or mud is pumped down the hollow drill string and out nozzles formed in the bit body. The drilling fluid or mud cools and lubricates the bit as it rotates and also carries material cut by the bit to the surface.
The bit body and other elements of earth-boring bits are subjected to many forms of wear as they operate in the harsh downhole environment. Among the most common form of wear is abrasive wear caused by contact with abrasive rock formations. In addition, the drilling mud, laden with rock cuttings, causes erosive wear on the bit.
The service life of an earth-boring bit is a function not only of the wear properties of the PDCs or cemented carbide inserts, but also of the wear properties of the bit body (in the case of fixed cutter bits) or cones (in the case of roller cone bits). One way to increase earth-boring bit service life is to employ bit bodies or cones made of materials with improved combinations of strength, toughness, and abrasion/erosion resistance.
Accordingly, there is a need for improved bit bodies for earth-boring bits having increased wear resistance, strength and toughness.